Tonight I finished the retrofit of GUS #1 (not the original - that would be Nicholas' GUS #0 of course) and managed to get a print of the official Simpson Test Squirrel:

Squirrel just completed

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Squirrel closeup

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I discovered a couple of things. First, I needed to measure and calculate my own calibration number. It turned out to be about 123 steps/mm, which is close to Nicholas' 120, but far enough off that I could not get a set of calibration points that would work with segmentize.py. Second, I have some kind of hysteresis in the printer, perhaps due to the spring not maintaining constant tension. I'm not entirely sure what's going on there, more testing will be needed. I'll probably fall back to 20 mm calibration cubes now that the official print has been made.

Incidentally, the combination of issues that we had with the original incarnation of GUS #1 meant that it never managed to make a complete squirrel, so we're already well ahead in that respect

I definitely have hysteresis in my system. It is not super noticeable when I fully tension. I will probably get stiff springs and really crank up the tension if I want to be a speed demon. These springs can only do about 3.5lbf so I am going to bump up to 10lbf max and give it a go.

Super exciting that you got a whole squirrel. Easier from here on out.

Considerable progress today; I was able to track down two causes of hysteresis in the arms. The first one that I resolved was contributing just a little, I think; the tension in the strings wasn't high enough. The springs would slightly extend and contract during the arm motion, and once I cranked up the tension to the point where they stayed extended, that error disappeared.

The other problem was harder to track down, and I finally figured it out by constructing some test gcode files that moved just one axis at a time. The Z axis, and to a lesser extent the X, had just barely loose pulleys, enough to let them turn by a few degrees. Of course, tightening the pulley screws with the arm fully assembled is very difficult, so I improvised a solution with a soldering iron:

Motor arm with access hole

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Once the pulleys were tight I re-did the calibration (fifth time, I think) and tried a 20 mm cube. The result wasn't perfect; the first layer is still dicey and there's definitely some skew, which Nicholas suspects is incorrect calibration on one or more of the arms. Nevertheless I tried a somewhat larger print, and got a pretty decent result:

Simpson prints a moai

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I intentionally chose something that would look okay even a little distorted, even though that's really cheating! Looks like one of those 12" calipers will be in order for the next stage of calibration. . .

I don't have any hex wrenches long enough to reach all the way in, the best I could do was work in between the halves of the gear, and that wasn't good enough to get the screws tight (as I discovered the hard way). I also didn't use nyloc nuts on the pulleys, but I'll probably change them.

The hole is small enough that I can't see anything except the center of the screwhead. I was actually thinking that a larger window in the side of the arm would be nice for watching the tracking but I wouldn't want to do anything that would reduce stiffness. I suppose I could reprint the arms in T-glase and just look through them